СПЕКТРОСКОПІЧНЕ ТА ТЕРМІЧНЕ ДОСЛІДЖЕННЯ НОВОГО ТЕОС–ГІДРОХІНОНОВОГО СИЛОКСАНОВОГО ОЛІГОМЕРУ: СТРУКТУРНА, ЕЛЕКТРОННА ТА ВОГНЕЗАХИСНА ХАРАКТЕРИСТИКА

A novel hydroquinone–formaldehyde organosilicon (siloxane) oligomer (HFTEOS) was synthesized by oxalic acid-catalyzed novolac condensation of hydroquinone with formaldehyde (2 : 2:1 molar ratio) followed by sol–gel condensation with tetraethoxysilane (TEOS). Structure and properties were characterized by FTIR, UV-Vis, DFT (B3LYP/6-311G), and TGA/DTA. FTIR confirmed retention of the hydroquinone aromatic skeleton (1510.45, 1458.76 cm⁻¹) alongside new Si–O–C(aryl) linkages (1190.27, 1095.51 cm⁻¹) and residual TEOS ethoxy groups (1009.36, 960.54, 930.39 cm⁻¹), indicating covalent bridging of the organic and inorganic phases; assignments were corroborated by a calculated harmonic IR spectrum. UV-Vis spectra, benchmarked against free hydroquinone (λmax = 194, 222, 288 nm), showed the corresponding K-band (~197–200 nm), secondary band (215–220 nm), and forbidden B-band (~275–280 nm), with no quinone-type oxidation above 320 nm. Geometry optimization of a 57-atom model fragment (C₂₀H₂₈O₈Si) revealed a distorted tetrahedral silicon centre (Si–O = 1.672–1.682 Å; O–Si–O = 104.7–116.2°) bridging two intact, non-quinoid hydroquinone rings through aryl-methyleneoxy arms, with two Si-bound oxygens remaining as unreacted ethoxy groups (Si–O–C ~129.0–130.9°), consistent with the mixed FTIR signature. HOMO–LUMO gaps narrowed along the series formaldehyde (5.82 eV) → hydroquinone (5.40 eV) → HFTEOS (4.90 eV), while Mulliken analysis showed the phenolic groups remain electronically near-identical to free hydroquinone. TGA/DTA revealed three decomposition stages (26–801 °C), with the main Si–O–C cleavage at 406.9 °C (ΔH = −17.3 J/g) and ~43 % residue at 800 °C. Fire-retardant testing of treated pine wood (GOST 16363-98; 10 % w/v ethanolic solution, 48 h impregnation) gave 0.268 kg/m² dry-substance consumption (30.4 kg/m³ total absorption) and 6.82 % mass loss on flame exposure, versus 65.1 % for untreated wood, placing the coating in fire-protection efficacy Group I. These results establish a coherent structure–property relationship supporting HFTEOS as an environmentally compatible, silicon-based wood fire retardant.